Indirect Calorimetry Critical in Identifying Early Hypometabolism and Late-Stage Hypermetabolism in Post-surgical Patients

ABSTRACT

Background: Indirect calorimetry (IC) is the gold-standard procedure for measuring resting energy expenditure (REE) in hospitalized patients. Predictive energy equations commonly use static variables and rarely account for changes in REE throughout hospitalization. We hypothesize that predictive equations are typically inaccurate in surgical intensive care unit (ICU) patients. More specifically, we hypothesize that predictive equations often overpredict measured resting energy expenditure (mREE) in early-stage critical illness and underpredict needs later in surgical ICU stay, leading to over-/under-feeding and associated complications. Method(s) This prospective observational trial enrolled surgical ICU patients who underwent emergent or urgent operations for abdominal trauma, perforated viscus, or ischemic bowel within 72 hours of their surgical procedure. Metabolic assessments were performed using the COSMED Q-NRG + Metabolic Monitor ventilator, mask, and canopy at regular intervals during and post ICU admission until hospital discharge. Measurements were categorized by post-surgical intervention ICU admission days 0-3, 4-7, 8-14, 15-21, and 22-28. Patients with multiple measurements taken during the same time interval were averaged. mREE reported in calories (kcal) per kilogram (kg) of admission body weight per day were compared in obese (BMI > 30 kg/m2) and non-obese (BMI < 30 kg/m2) subgroups. Compared to IC, the Mifflin St Jeor (MSJ) equation determined predicted REE using ICU admission anthropometrics. Data are reported as mean+/-standard error of the mean (SEM) and median (interquartile range), and a two-sided p-value of <0.05 was determined significant. Result(s) In total, 18 surgical ICU patients who contributed 47 IC measurements were included in the analysis (Table 1). Most measures were obtained within the first 7 days of post-surgical ICU admission (72%). mREE peaked between days 8-14 in obese and non-obese subgroups (20.6 vs 28.5 kcal/kg;p = 0.02) and was lowest during 0-3 days of post-surgical ICU admission in both groups. Across all 5-time intervals, average kcal/ kg ranged from 14.7-20.6 among obese patients and from 20.1-28.5 in non-obese counterparts (Table 2). Non-obese patients had higher mREE per kg of body weight than obese patients at all time points (Figure 1). MSJ over-predicted mREE during the first 7 days post ICU admission in non-obese patients and within the first 3 days in obese patients and underpredicted mREE in both groups thereafter. Conclusion(s) Equations such as MSJ over- and under-predict mREE in post-operative surgical ICU patients depending on the days elapsed since post-surgical ICU admission. ASPEN's current guideline recommendation of 12-25 kcal/kg may also underfeed post-surgical populations while 25 kcal/kg may not support hypermetabolism among non-obese patients seen in week 2 following post-surgical ICU admission. Alternatively, MSJ multiplied by a 1.2 activity factor may account for hypermetabolism during this time. Notably, non-obese patients experienced greater hypermetabolism than obese patients during week 2 which is consistent with our previously published data in mechanically ventilated COVID- 19 patients. Additionally, the striking dichotomy between the mREE of obese and non-obese patients at all post-surgical time points should be considered in the clinical care of patients. Ultimately, IC remains the gold-standard means of measuring REE and is a critical tool to capture the dynamic nature of energy requirements in post-surgical populations as weight-based and predictive equations continually fall short. (Table Presented).